In its broadest aspect the present invention relates to U-shape and/or nozzle-U-loop fermentors and methods of the operation of the same.
More specifically the invention relates to U-shape and/or nozzle-U-loop fermentors and methods for the operation of the same, which are particularly appropriate for production processes with methanotrophic bacteria and similar processes, whereby different gases and other nutrients are to be supplied to the fermentation liquid in order to obtain an optimally proceeding fermentation process with the highest possible yield of fermentation product in the shortest possible period of time.
Stirred Or Back-mix Fermentors
In conventional bioreactors (fermentors) the mixing of gases with the fermentation liquid is effected by means of stirrer blades placed centrally in the fermentor. The stirrer blades generate turbulence in the liquid, which means that gas, usually injected at the bottom of the reactor, will be dissipated in the liquid in the form of small fine gas bubbles. This type of reactor provides a relatively homogenous mixing, i.e. that about the same concentrations of gases and substrates will be found whether measuring at the top or at the bottom of the reactor. This type of reactor is, however, not particularly appropriate for up-scaling, since it is difficult to obtain the same homogeneous mixing and the same mass transportation in large reactors as can be obtained in small laboratory and pilot reactors. Besides, the vigorous mixing implies a significant heating of the fermentation liquid.
Airlift and Loop Fermentors
In order to avoid the mechanical stirring, different types of airlift reactors have been developed. The majority of these reactors are so-called loop reactors having two sections: an upstream part and a downstream part, which are interconnected at both ends. Gases are supplied at the bottom of the reactor in the upstream part in an arrangement, which yields small gas bubbles (e.g. through a vitrified ceramic plate or an array of small nozzles). The bubbles mix with the liquid whereby the total density is reduced and the gas-liquid mixture ascends displaced by new liquid emerging from the downstream part. The gas-liquid mixture moves up through the upstream part of the reactor and releases its gas bubbles at the top, whereupon the liquid descends down through the downstream part. In order to obtain a long residence time for the gas bubbles in the liquid, airlift reactors are conventionally tall slender reactors. This implies that the gas must be supplied at a high pressure for overcoming the hydrostatic pressure at the bottom of the reactor. If the gas is air, this implies the use of compressors. Furthermore, airlift reactors have a relatively poor exploitation of the injected gas. Typically only 20-40% of the gas is utilized. Besides, it is difficult to obtain good and quick release of the gas bubbles from the fermentation liquid at the top of the reactor and separation of the gas phase thus produced (which may be rather foaming) from the liquid phase before the fermentation liquid moves down in the downstream part of the reactor.
U-shape Reactor
The U-shape reactor is constructed with a view to provide:
Non-compressed or nearly non-compressed gas injection
Long residence time and thus high degree of exploitation of the injected gas
Low energy consumption for liquid circulation
Simple design
Good separation of gases and liquid at the top.
In principle the U-shape reactor is also a loop reactor. However, contradictory to conventional loop reactors the liquid circulation is effected by means of one or more in-line pumps. This (or these) pump(s) may be of the propeller pump type, wherein the propeller blades are designed for pumping a mixture of liquid and gas. The gases can be introduced at different locations in the U-shape loop, but typically they will be supplied at the upper end of the downstream part of the loop. By introducing the gases at the upper end of the downstream part of the loop a nearly non-compressed injection is obtained, since the gases only have to overcome a hydrostatic pressure of some few meters. The gases can be introduced by means of particular gas dispensers providing for a distribution across the downstream part of the loop. Fine dispersion of the gases in the liquid is effected by means of static mixing elements placed immediate below the gas injectors (the mixing elements may be of e.g. Sulzer manufacture). The liquid flow in the downstream part of the loop must be sufficiently high so that all the injected gas is carried along down through the static mixers. Here a comminution of the gas is effected so that a large number of small gas bubbles is obtained, which are dispersed uniformly in the liquid. The bubbles are carried along with the liquid flow down through the downstream part of the loop to its lower end and further on through a U-bend to the upstream part of the loop so that the gas bubbles are redispersed (e.g. by means of static mixing elements) several times in the liquid.
The U-bend causes a centrifugal effect and thus some separation of gas bubbles and liquid.
Therefore, the in-line pump is preferably placed adjacent the U-bend, partially because it then assists in producing a redispersing of the gas in the liquid and partially because it is practical to have it placed at the bottom of the fermentor.
In order to obtain a good bubble distribution in the upstream part of the loop more static mixers may be provided therein.
The top of the fermentor is designed so that the upstream part of the loop via a bend is passed horizontally onto the side of a widening of the upper end of the downstream part of the loop. This particular construction feature assists in yielding a good separation of liquid and gas bubbles, as centrifugal forces act in the bend and in the very widening of the upper end of the downstream part of the loop a vigorous circulation of the liquid with corresponding accompanying centrifugal forces arise, which also bring about separation of liquid and gas bubbles. Thereby, one of the great problems associated with airlift reactorsxe2x80x94viz. separation of the gas and liquid phasesxe2x80x94is solved in an utmost elegant fashion.
Furthermore, the U-shape reactor provides for a long contact time between the gas and liquid phases, as the injected gas is present both in the downstream and in the upstream parts of the loop. This means that an essentially higher utilization of the gas is obtained compared with conventional airlift reactors.
Gas bubbles in liquids have a tendency to fuse together to larger volumes (coalesce). This tendency contributes to making conventional airlift reactors ineffective inasmuch as the bubbles become larger and larger upward through the upstream part, partly due to coalescence and partly due to a reduced hydrostatic pressure. In the U-shape reactor here described, this tendency in the upstream part is counteracted by providing static mixers appropriately spaced apart at distances, which depend on the medium applied. In the downstream part, the increasing hydrostatic pressure counteracts the tendency to increased bubble sizes. To the extent that this effect cannot balance the fusion (coalescence) of the gas bubbles there is provided for a redispersing by means of static mixers.
The amount of gas, which advantageously can be dispersed in the liquid, depends on the hydrostatic pressure. In the case of tall reactors it will therefore be advantageous to have several locations for the introduction of gases in the downstream part. The only requirement to the gas inlets is that at least one static mixing element is placed immediately after each inlet for dispersing the gas in the liquid.
In order to give an impression of the dimensions, which such a U-shape reactor may have it may be mentioned that its total height can be about 40 metres and its width can be about 6.6 metres, the said width is to be understood as the perpendicular distance between the portions of the vertical walls of the downstream and upstream parts being spaced furthest from each other. The internal diameter, d, of the downstream part and the upstream part, respectively, can be about 1.65 metre, and the radius of the bend part at the ends of the downstream part 2 and the upstream part 4 can be 1.5xc3x97d.
U-shape and/or nozzle-loop fermentors of the above type are disclosed in DK patent No. 163066 (EP-B-0 418 187). These fermentors are i.a. well suited for use in production processes with methanotrophic bacteria.
Production processes with methanotrophic bacteria are anaerobic and based on natural gas as carbon and energy sources. Atmospheric air, pure oxygen or atmospheric air enriched with pure oxygen is used as oxygenation source in the fermentation process and ammonium is used as nitrogen source. In addition to theses substrates the cultivation of methanotrophic bacteria requires water, phosphate, and several minerals such as magnesium, calcium, potassium, iron, copper, zinc, manganese, nickel, cobalt and molybdenum. Sodium hydroxide and sulphuric acid are used for pH adjustments. All chemicals are food grade. Phosphate is supplied in the form of phosphoric acid, minerals as sulphates, chlorides or nitrates. The pH value is controlled to 6.5xc2x10.3 and the temperature is maintained at 45xc2x0 C.xc2x12xc2x0 C.
Methanotrophic bacteria are produced by continuous fermentation. A nozzle-loop fermentor with static mixers is used. The nozzle-loop fermentor brings about high utilization of the gases carried along through the loop with the approximately plug-flowing fermentation liquid, The gases are supplied at the beginning of the loop and stay well admixed with the liquid until their separation off at the headspace at the end of the loop.
Injection in the fermentor can be effected at one or more locations, e.g. at four placesxe2x80x94at top, at the middle of the down loop, at the bottom of the down loop, and after the passage of the U-bend part at the bottom a small distance above that. In all circumstances the gases are supplied in advance of the mechanical mixer(s), which is (are) placed immediately after each injection in the flow direction of the fermentor, cf. the figure of the drawing.
Additionally, more mechanical mixers may be placed elsewhere in the fermentor.
The drawback of the prior art U-shape and/or nozzle-U-loop fermentors in connection with the above mentioned production processes with methanotrophic bacteria and corresponding processes, wherein different gases are to be continuously supplied, which partly may be expensive and partly may constitute a potential danger of explosion if they accumulate in major amounts in the reactor, is that up to now it has been difficult or even impossible to supply these gases as well as the additional nutrients (ammonium, phosphate and minerals) necessary for the fermentation process, and pH controlling means in such amounts and relative ratios that it has been possible to obtain an optimum utilization of the gases before the separation off in the headspace of the reactor with simultaneous achievement of an optimally proceeding fermentation process providing the largest possible yield of fermentation product in the shortest possible time. This is due to the fact that hitherto it has been necessary to run the addition of the above mentioned process substances to the reactor on the basis of a predetermined dosage-time schedule which has been worked out on the basis of previously performed test runs during which samples of the fermentation liquid have been taken, which samples subsequently have been analysed for relevant constituent substances in the laboratory. The fermentation processes are, however, biological processes which proceed far from uniformly from time to time, but are subjected to even very large variations for which reason the time-schedule fixed doses will not correspond to the actual need for attainment of the results aimed at with respect to the optimum utilization of the gases before the separation thereof in the headspace of the reactor and attainment of the largest possible yield of fermentation product in the shortest possible time.
The present invention provides a U-shape and/or nozzle-U-loop fermentor and a method of performing a fermentation process, wherein the above drawbacks are avoided and, thus, it becomes possible to supply necessary gases and the additional nutrients required for the fermentation process, pH adjustment means and water in such amounts and ratios that at all times it corresponds to the actual need for achieving an optimum utilization of the gases before separation thereof in the head-space of the reactor simultaneous with obtaining an optimally proceeding fermentation process with the largest possible yield of fermentation product in the shortest possible period of time.
This object is achieved with the U-shape and/or nozzle-U-loop fermentor according to the invention, which has a U-part consisting of an essentially vertical downstream part 2, an essentially vertical upstream part 4, a U-shape bend part 3 which connects the lower ends of said downstream and upstream parts, an in-line pump 12 placed in the U-part for the circulation of fermentation liquid in the fermentor, a top part 5 placed above the upper end of the downstream part and having the form of a cylinder with a diameter which is substantially larger than the diameter of the downstream part and being connected thereto via a truncated cone-shaped connection member, whereas the upper end of the upstream part 4 via a bend is passed substantially horizontally and tangentially into the lower part of the top part 5, a vent pipe 6 for exhausting the gas(es) separated in the head-space of the top part, an outlet 11, preferably placed in the U-bend part 3, for draining off fermentation liquid, and gas supply means 7,8,9,10, which according to wishes and needs optionally are placed in the downstream part, the U-bend part, and the upstream part, preferably in the lower end thereof, with accompanying static-mechanical mixing members 13,14,15,16,17 for the comminution of the gases introduced into the fermentation liquid, and inlet means for water and nutrient salts 18 and 19, respectively, said fermentor being characterized in that ion sensor(s) or analyser(s) 20,21,22,23 for sensing the concentration of at least one of the ion species phosphate, ammonium, nitrate and hydrogen ion, oxygen sensor(s) for sensing the oxygen concentration and at least one thermo phial for sensing the temperature are provided in-line in the circulating fermentation liquid in connection with the supply means 7,8,9,10,18,19 or in by-pass arrangements attached thereto, said sensor(s), analyser(s) and phial(s) delivering signals to a data processing system (PC) wherein the received signals are processed and the doses of supplied gases, water, minerals and pH controlling means supplied via the supply means 7,8,9,10,18,19 are calculated and optimised from pre-programmed amounts in relation to the results measured.
From the outlet 11 the fermentation liquid with its content of biomass and dissolved gases, etc. are pumped to a gas separator from which separated residual gases are recirculated to the fermentor, whereas the fermentation liquid is passed to a separator (centrifuge) for up-concentration of the content of solids in the fermentation liquid and from there further on to a sterilization unit and an ultra filtration unit eventually to end up in a spray drying unit, wherein the product solids of the fermentation are recovered, whereas the amounts of liquid separated in these units with their contents of nutrients essentially are recirculated to the fermentor loop.
The invention also relates to a method of performing a fermentation process, by which method water, fermenting micro-organisms, at least two different gases, necessary nutrient salts and pH controlling means as well as possible recovered fermentation liquid (supernatant) are introduced into a U-shape and/or nozzle-U-loop fermentor and fermentation liquid is withdrawn, the fermentation liquid being circulated in the fermentor by means of an in-line pump placed in the U-part of the fermentor, said method being characterized in that the concentration of at least one of the ion species phosphate, ammonium, nitrate and hydrogen ion are sensed by an ion sensor or analyser which is placed in-line in the circulating fermentation liquid in connection with supply means for gases, nutrient salts, pH controlling means and water, in that the oxygen concentration of the fermentation liquid is correspondingly sensed by an oxygen sensor being placed in connection with the respective supply means, and in that the temperature of the fermentation liquid is sensed by at least one thermo phial, the said sensors, analysers and phials delivering signals to a data processing system (PC), wherein the signals received are processed, and the doses of supplied gases, water, minerals and pH controlling means added via the supply means are calculated and optimised from pre-programmed amounts in relation to the results measured.
Preferably, both phosphate, ammonium and nitrate are measured in-line with an ion analyser at the injection of the gases or gas-liquid mixture, and oxygen is also measured in-line with an oxygen sensor. Furthermore, also pH values and temperature are measured in-line.
It is a common feature for all measures and gas injections and other substances of addition that they are controlled and adjusted on the basis of the results measured in the fermentation liquid, which are transferred to a data processing system (PC), wherein the data are processed and the doses of addition via the nozzle arrangements are calculated and optimised from pre-programmed amounts relative to the results measured.
The nozzle arrangement can be a single flow or a multiple flow arrangement. By multiple flows it will be liquid from the fermentor, which is mixed with gas, either oxygen, natural gas or any other gas, before injection under pressure in the fermentor, so that a fine dispersion is obtained.
In case of a multiple flow arrangement ammonium, phosphate and nitrate can be measured via a by-pass in the return flow, e.g. with an ion analyser, and the oxygen content can be measured on-line via other measuring instruments, while other ingredients are monitored via laboratory analyses.
The pressure in the nozzles can be varied so that the dispersion (bubble size) can be measured in the liquid or via a high-speed camera placed in connection with the injection and designed so that switching between the different nozzle arrangements can be effected according to a programmed cycle.
The fermentors are either without pressure or have a constant gauge pressure above that of the atmosphere. The pressure is here controlled via the consumption of the gases injected into the head-space of the fermentor so that a variable counter pressure can be established relative to the residual gases measured in the head-space, and the consumption of the gases is supervised and controlled so that the head-space at no point of time is filled with a mixture of gases which constitutes an explosion risk.
Thus, in operation the fermentor can be without any over pressure in the headspace or with an over pressure of up to 2-3 bars or more.
Similar measurements are performed on the supernatant, which is returned from the centrifugal separation, and on the liquid, which is passed back from ultra filtration, so that back conveyance of these liquids with the contents they may have of different organic and inorganic substances are also incorporated in the optimisation of the fermentor.
No other fermentor, it being a U-nozzle loop or a stirred fermentor, has an optimisation process as that disclosed herein, via on-line measurements, including ion analysers, and optimisation of gas injections and other substance additions via nozzle arrangements, wherein the pressure can be varied relative to the optimum consumption of the gases and security conditions.